Desorption of gold nanoclusters (2-30 nm) under low excitation of their electronic subsystem by Ar ions (14 keV/nm)

Gold nanodispersed targets with islands-grains sized 2-30 nm were irradiated by Ar⁷⁺ ions with the energy of 45.5 MeV and (dE/dx)_e = 14.2 keV/nm in gold. The desorbed gold nanoclusters were studied by TEM method. For all the targets desorption of intact gold nanoclusters is observed. However, for inelastic stopping of monatomic Ar ions in gold of 14.2 keV/nm desorption of nanoclusters is observed only up to ∼25 nm. The yield of the desorbed nanoclusters considerably decreases from 3 to 0.02 cluster/ion with the increase of the mean size of the desorbed nanoclusters from 3 to 14.2 nm. The results are discussed.     

Desorption of gold nanoclusters from gold nanodispersed targets by 200 keV Au5 polyatomic ions in the elastic stopping mode: Experiment and molecular-dynamics simulation

Au nanoislet targets (∅ 2-60 nm) were bombarded by 200 keV polyatomic ions (40 keV/atom), which deposit their energy mainly in the nuclear stopping mode: ∑(dE/dx)_n = 30 keV/nm and ∑(dE/dx)_e = 2 keV/nm. The matter desorbed in the form of nanoclusters was registered by TEM. The total transfer of matter was determined by neutron-activation analysis. The total yield of the ejected gold reached high values of up to 2.6 × 10⁴ atoms per Au₅ ion. The major part (2 × 10⁴ atoms per ion Au₅) of the emission is in the form of nanoclusters. The results are compared with the data of similar experiments with 1 MeV Au₅ (200 keV/atom) and other projectiles. The analysis of the experimental data and the comparison to molecular-dynamics simulation results of the desorption process show that the desorption of Au nanoislets is induced by their melting, build-up of pressure and thermal expansion.     

New local model for electronic energy loss and its application to computer simulations of channeling

We used the average of the Thomas-Fermi (TF) electron distribution instead of that of Hartree-Fock (HF) electron distribution as the screening length of an isolated atom. Based on the Firsov theory, we proposed a new Firsov formula of the electronic energy loss which has a simple form ΔE_e(E, b) ∞ S_e(E) exp(γb)/(1 + βb)⁶, where S_e(E) is the electronic stopping cross section, b = p/a, p and a are the impact parameter and the screening length, respectively, and β and γ are the fitting parameters. Using the present screening lengths with the shell effect and the new Firsov formula, the depth distributions of channeling were simulated by the ACOCT code for 20 keV B⁺ ions impinging along the [1 1 0] channel direction of silicon (1 1 0) surface. The ACOCT depth profiles of channeling using the new Firsov (solid) local model for the AMLJ potential are in good agreement with the experimental ones.     

Formation of planar waveguide in BiB3O6 crystal by MeV carbon implantation

Non-leaky planar waveguide structure has been fabricated in x-cut BiB₃O₆ crystal by 6 MeV C³⁺ ion implantation at a dose of 1 × 10¹⁴ ions/cm². The effective refractive indices of the waveguide are measured at a wavelength of 632.8 nm. We perform a computer code based on the finite difference method to reconstruct the refractive index profiles of n_x and n_y of this waveguide. The beam propagation method is used to calculate the electric and magnetic field profiles in the waveguide region from the reconstructed refractive index profiles. Our simulated data show that the refractive index increased waveguide layer can confine the mode completely.     

Rydberg and autoionizing triplet states in Helium up to the N = 5 threshold

Energy levels of highly excited bound Rydberg states, the position and widths of autoionizing states, and oscillator strengths are calculated for He ³S, ³P^e, ³P^o, ³D^e and ³D^o symmetries up to the N = 5 He⁺ excitation threshold. The calculations are performed with the K-matrix B-spline method with maximum orbital angular momentum ?_max = 8. Reliable doubly excited-state parameters up to the n = 20 multiplet below each ionization threshold are presented. One thousand and six hundred newly identified bound and metastable states, seven times those available in literature, fill many gaps, reveal a dozen intruder states, and allow new speculations on propensity rules and radiative decays of triplet Rydberg states.     

Modification of the etching properties of x-cut Lithium Niobate by ion implantation

In this work x-cut Lithium Niobate crystals were implanted with 0.5 MeV O ions (nuclear stopping regime), 5 MeV O ions (sub-threshold electronic stopping regime) and 12.5 MeV Ti ions (ion track regime) at the fluences required for the formation of a surface fully disordered layer. The damage depth profiles were determined by RBS-channeling. Wet etching was performed at room temperature in 50% HF:H₂O solution. The data indicated an exponential dependence of the etching rate on the damage concentration. Independently of the damage regime, once random level in the RBS-channeling spectra was attained we measured the same etching rate (50-100 nm/s) and the same volume expansion (∼10%) in all samples. These results indicate that the fully disordered layers obtained by electronic damage accumulation have the same chemical properties of those obtained by conventional nuclear damage accumulation and therefore they can be defined “amorphous”. The impressive etching selectivity of ion implanted regions makes this process suitable for sub-micro machining of Lithium Niobate.     

Elastic electron scattering by silver atom

The experimental and theoretical studies of elastic electron scattering by silver atom have been carried out. The experimental investigation was based on crossed beam technique with effusive atomic beam being perpendicularly crossed by electron beam. The measurements were performed at electron-impact energies (E₀) of 10, 20, 40, 60, 80 and 100 eV and for a range of scattering angles (θ) from 10° up to 150°. The absolute differential cross sections (DCSs) have been obtained from the elastic-to-inelastic (the unresolved silver resonant lines 4d¹⁰5p²P_{1/2, 3/2}) intensity ratio at θ = 10° at each E₀. Calculations have been performed using the parameter-free complex optical potential (OP) with the inclusion of spin-orbit interaction for the same E₀. Comparison between present experiment and theory has been made.     

Radiolysis studies of aqueous κ-carrageenan

The effects on N₂O and N₂ gas on the radiation degradation yield of aqueous kappa (κ-) carrageenan were investigated. The G_d of solution saturated with N₂O solution was expectedly much higher than in air (1.7 and 1.2 × 10⁻⁷ mol J⁻¹). On the other hand, a lower G_d of 1.1 × 10⁻⁷ mol J⁻¹ was obtained from κ-carrageenan solution saturated with N₂.The rate constant of reaction of OH radicals with sonicated and irradiated κ-carrageenan were determined using e-beam pulse radiolysis. The rate constant of OH interaction with sonicated κ-carrageenan decreased with decreasing molecular weight. On the other hand, the OH interaction with irradiated κ-carrageenan decreased but did not vary significantly with decreasing molecular weight. Metal ion (Na⁺) induced conformational transition into helical form decreased the rate constant of OH reaction with κ-carrageenan. Likewise, the G_d in aqueous form was affected by the conformational state of κ-carrageenan. The helical conformation gave a lower G_d (7 × 10⁻⁸ mol J⁻¹) than the coiled conformation (G_d = 1.2 × 10⁻⁷ mol J⁻¹).     

Effects of dissolved oxygen on electrochemical and semiconductor properties of 316L stainless steel

The effects of dissolved oxygen on the electrochemical behavior and semiconductor properties of passive film formed on 316L SS in three solutions with different dissolved oxygen were studied by using polarization curve, Mott-Schottky analysis and the point defect model (PDM). The results show that higher dissolved oxygen accelerates both anodic and cathodic process. Based on Mott-Schottky analysis and PDM, the key parameters for passive film, donor density N_d, flat-band potential E_fb and diffusivity of defects D₀ were calculated. The results display that N_d(1−7 × 10²⁷ m⁻³) and D₀(1−18 × 10⁻¹⁶ cm²/s) increase and E_fb value reduces with the dissolved oxygen in solution.     

Swelling and optical properties of Si3N4 films irradiated in the electronic regime

Silicon nitride layers of 140 nm thickness were deposited on silicon wafers by low pressure chemical vapour deposition (LPCVD) and irradiated at GANIL with Pb ions of 110 MeV up to a maximum fluence of 4 × 10¹³ cm⁻². As shown in a previous work these irradiation conditions, characterized by a predominant electronic slowing-down (S_e = 19.3 keV nm⁻¹), lead to damage creation and formation of etchable tracks in Si₃N₄. In the present study we investigated other radiation-induced effects like out of plane swelling and refractive index decrease. From profilometry, step heights as large as 50 nm were measured for samples irradiated at the highest fluences (>10¹³ cm⁻²). From optical spectroscopy, the minimum reflectivity of the target is shifted towards the high wavelengths at increasing fluences. These results evidence a concomitant decrease of density and refractive index in irradiated Si₃N₄. Additional measurements, performed by ellipsometry, are in full agreement with this interpretation.     

Electronic stopping dependence of ion beam induced modifications in GaN

We report here Swift heavy ion induced effects in GaN samples grown by metal organic chemical vapor deposition (MOCVD) technique. These samples were irradiated with 80 MeV Ni and 100 MeV Ag ions at a fixed fluence of 1 × 10¹³ ions/cm². Ion species and energies are chosen such that the difference in their electronic energy loss (S_e) would be 8 keV/nm. Effects of Ag on structural and optical properties over Ni ions have been discussed extensively. We employed different characterization techniques like High Resolution X-ray Diffraction (HRXRD) and Raman Spectroscopy for defect density calculations and for vibrational modes, respectively. Defect densities are calculated and compared using Williamson-Hall method from HRXRD. Change of strain and vibrational modes with S_e has been discussed.     

Ion-induced electron emission monitoring of the structure and morphology evolution in HOPG

The temperature dependences of the ion-induced electron emission yield γ of highly-oriented pyrolytic graphite (HOPG) under high-fluence (10¹⁸-10¹⁹ ions/cm²) 30 keV Ar⁺ ion irradiation at ion incidence angles from θ = 0^o (normal incidence) to 80^o have been measured to trace both the structure and morphology changes in the basal oriented samples. The target temperature has been varied during continuous irradiation from T = −180 to 400 ^oC. The surface analysis has been performed by the RHEED and SEM techniques. The surface microgeometry was studied using laser goniophotometry (LGF). The dependences of γ(T) were found to be strongly non-monotonic and essentially different from the ones for Ar⁺ and N₂⁺ ion irradiation of the polygranular graphites. A sharp peak at irradiation temperature T_p ≈ 150 ^oC was found. A strong influence of electron transport anisotropy has been observed, and ion-induced microgeometry is discussed.     

Energy levels, transition probabilities, and electron impact excitations for La XXX

The energy levels, spontaneous radiative decay rates, and electron impact collision strengths are calculated for La XXX. The data refer to 107 fine-structure levels belonging to the configurations (1s²2s²2p⁶)3s²3p⁶3d¹⁰, 3s²3p⁶3d⁹4l, 3s²3p⁵3d¹⁰4l, and 3s3p⁶3d¹⁰4l (l = s, p, d, f). The collision strengths are calculated with a 20-collision-energy grid in terms of the energy of the scattered electron between 10 and 10,000 eV by using the distorted-wave approximation. Effective collision strengths are obtained at seven electron temperatures: T_e (eV) = 10, 100, 300, 500, 800, 1000, and 1500 by integrating the collision strengths over a Maxwellian electron distribution. Coupled with these atomic data, a hydrodynamic code MED103 can be used to simulate the Ni-like La X-ray laser at 8.8 nm.     

Energies, wavelengths, and transition probabilities for Ge-like Kr, Mo, Sn, and Xe ions

Energy levels, wavelengths, transition probabilities, and oscillator strengths have been calculated for Ge-like Kr, Mo, Sn, and Xe ions among the fine-structure levels of terms belonging to the ([Ar] 3d¹⁰)4s²4p², ([Ar] 3d¹⁰)4s 4p³, ([Ar] 3d¹⁰)4s²4p 4d, and ([Ar] 3d¹⁰)4p⁴ configurations. The fully relativistic multiconfiguration Dirac-Fock method, taking both correlations within the n=4 complex and the quantum electrodynamic effects into account, have been used in the calculations. The results are compared with the available experimental and other theoretical results.     

The inhibiting effects of hydrogen on the corrosion of uranium dioxide under nuclear waste disposal conditions

A number of electrochemical experiments were employed to investigate the effects of hydrogen on the corrosion of UO₂ under nuclear waste disposal conditions. A combination of corrosion potential (E_CORR) measurements and cyclic voltammetry have indicated that dissolved hydrogen can polarize the UO₂ surface to reducing potentials; i.e., to E_CORR values more negative then those observed under anoxic (argon-purged) conditions. A comparison of the behaviours of SIMFUEL specimens with and without incorporated noble metal ε-particles indicates that these particles may act as catalytic electrodes for H₂ oxidation, H₂ ↔ 2e⁻ + 2H⁺. It is the galvanic coupling of these particles to the UO₂ matrix which suppresses the fuel corrosion potential.     

Theoretical transition probabilities, oscillator strengths, and radiative lifetimes of levels in Pb IV

Transition probabilities and oscillator strengths of 176 spectral lines with astrophysical interest arising from 5d¹⁰ns (n = 7,8), 5d¹⁰np (n = 6,7), 5d¹⁰nd (n = 6,7), 5d¹⁰5f, 5d¹⁰5g, 5d¹⁰nh (n =  6,7,8), 5d⁹6s², and 5d⁹6s6p configurations, and radiative lifetimes for 43 levels of Pb IV, have been calculated. These values were obtained in intermediate coupling (IC) and using relativistic Hartree-Fock calculations including core-polarization effects. For the IC calculations, we use the standard method of least-square fitting from experimental energy levels by means of the Cowan computer code. The inclusion in these calculations of the 5d¹⁰7p and 5d¹⁰5f configurations has facilitated a complete assignment of the energy levels in the Pb IV. Transition probabilities, oscillator strengths, and radiative lifetimes obtained are generally in good agreement with the experimental data.     

Electron collisions with Fe-peak elements: Forbidden transitions between the low lying valence states 3d, 3d4s, and 3d4p of Fe III

Effective collision strengths are presented for the Fe-peak element Fe III at electron temperatures (T_e in degrees Kelvin) in the range 2 × 10³ to 1 × 10⁶. Forbidden transitions results are given between the 3d⁶, 3d⁵4s, and the 3d⁵4p manifolds applicable to the modeling of laboratory and astrophysical plasmas.     

Thermal spike analysis of ion-induced tracks in semiconductors

Track data reported in InP and GaAs are analyzed according to the analytical thermal spike model (ATSM) and good agreement with the predictions is found. The Gaussian width of the thermal spike is a(0) ≈ 11 nm compared to a(0) = 4.5 nm in insulators. When the ion velocity v_p is high (E > 8 MeV/nucleon), a similar fraction of the electronic stopping power S_e is transformed into thermal energy of the spike in insulators and semiconductors. The results show that - compared to insulators - v_p affects only slightly the track sizes in semiconductors, which is explained qualitatively by the Coulomb explosion mechanism. The reported correlation between the bandgap energy E_g and a(0) is completed with new data. The results of previous analyses of ion-induced tracks in InP by ATSM are discussed.     

Measurement of thermal neutron and resonance integral cross sections of the reaction V(n,γ)V using a 20 Ci Am–Be isotopic neutron source

The thermal neutron capture cross section (σ_o) and the resonance integral (I_o) of the ⁵¹V(n,γ)⁵²V reaction were measured with an activation method to provide fundamental data for reactor calculation, activation analysis, and other theoretical and experimental uses concerning the interaction of neutron with matter. The vanadium and manganese samples were irradiated within and without a Cd shield case using a 20 Ci Am–Be neutron source. The activities of the samples were measured using gamma-ray spectroscopy. The thermal neutron capture cross section and the resonance integral were determined relative to the reference reaction ⁵⁵Mn(n,γ)⁵⁶Mn and the values obtained are 5.16 ± 0.19 barns and 2.53 ± 0.1 barns respectively. The previous measurements of the σ_o and I_o of the reaction ⁵¹V(n,γ)⁵²V were reviewed and the difference between the present values and the previous results were discussed.